Combination of PVC and Other Additives to Improve Cards: Case History of Special Grade PVC
PVC Basics
Polyvinyl chloride, now commonly known as PVC, is a member of the ethylene
family of polymers. The chemical process for making such polymers involves
taking the simplest unit, called the monomer, and linking these monomer
molecules together in the polymerisation process. Long molecular chains
are formed called polymers. Vinyl chloride (CVM), the monomer from which
PVC is made, was first synthesised in the laboratory by Justus von Liebig
in 1835. PVC itself was synthesised by Baumann in 1872 but it was not
until the late 1920s that the first commercial production of PVC took
place in the USA. Large-scale production in Europe followed during the
next two decades.
In simple terms, passing an electric current chemically decomposes
salt dissolved in water. This produces chlorine, caustic soda and hydrogen.
The oil or gas is refined and cracked to give ethylene. When the ethylene
and chlorine are combined, the product is dichloro-ethane; this can
again be transformed to produce vinyl chloride, the basic building block
of polyvinyl chloride or PVC.
The process of "polymerisation" links together the vinyl
chloride molecules to form chains of PVC. The PVC produced in this way
is in the form of a white powder. Most commodity plastics have carbon
and hydrogen as their main component elements. PVC differs by containing
chlorine (around 57 percent by weight) as well as carbon and hydrogen.
The chlorine content also helps to make PVC flame retardant. PVC polymer
is chemically stable, neutral and non-toxic.
This is not used alone, but blended with other ingredients to give
formulations for a wide range of products. PVC formulations have a wide
range of applications including the most sensitive, such as medical
equipment, construction, toys, automotive and electrical cabling, and
plastic cards.
Today PVC has become the second largest commodity plastic after polyethylene,
unquestionably the main raw material for plastic cards. The creation
of a good substrate for credit cards begins with the choice of the best
ingredients, first of all the PVC, the base element every formulation.
Polymerisation
Although the polymerisation reactions are many (mass, emulsion and
suspension), for aspects connected to the transformation process (calendering)
and to the application (credit cards), the tendency is for suspension.
The "suspension" reaction guarantees a better control of the
parameters of reaction than "mass" and higher purity of the
resin than "emulsion".
The reaction conditions have strong influence on the characteristics
of the resin and in particular on some parameters closely correlated:
K value, grain size, porosity and density. For example K value, measurement
of the molecular weight, is strongly influenced in inverse proportionality,
by the temperature of reaction.
The dimension of the resin and its homogeneity are critical elements
to obtain a homogenous mixture of product; normally the dimensional
target is between 120 and 150 microns with a narrow distribution around
to the average value. Within such values a homogenous gelation and constant
quality of the product are guaranteed. The use of lower values could
have handling implications while higher values would lead to major difficulties
in obtaining a homogenous product.
Porosity and density complete the picture. A too porous resin (low
density) will render the liquid components of the formulation (i.e.
lubricants) less effective because of too rapid absorption rate; and
moreover the lightness of powders complicates the product handling in
the phases of transport/transfer and mixing.
Raw material selection
The choice of the appropriate resins is limited due to some constraints
during the calendering process as well as in the finished product. Also,
due to the limits of the processing of PVC products, we cannot forget
that the calendering process demands high temperatures and therefore
the product comes into contact with the cylinders that normally have
a temperature between 160-200¦ C (320-392¦ F). PVC will stick on the
cylinders and, as a direct consequence of this, the management of the
viscosity of the material between the cylinders is more critical.
Simplifying these two conditions is the main target from the card manufacturing
point of view: good printing surface and excellent physical and mechanical
properties. PVC is hard, rigid, has good ink affinity, is clear, and
the presence of chlorine in the molecule makes PVC particularly versatile
as it makes it compatible with a wide range of other materials. On the
other hand, PVC has low temperature resistance and therefore has easy
degradation, poor mechanical properties (low elasticity and resilience)
and if these were not enough, once calendered, the PVC power becomes
yellow. However, because of its versatility, there are a lot of possibilities
to modify these characteristics by blending the PVC with the suitable
additives. It isn't particularly easy.
In truth, the card manufacturing requirements are not limited to the
two aspects that we have identified before, good printing surface and
excellent physical and mechanical properties — as the complexity
of the process of transformation of sheets of PVC in plastic cards is
much more demanding. All the parameters, both physical and mechanical,
must be consistent within the same production batch and in subsequent
batches and particular attention is paid to the roughness, colour, gloss,
opacity, printability and ink adhesion because the card manufacturing
process doesn't finish after printing.
The mechanical properties are even more important, therefore elasticity,
resilience, softness and flexibility are all taken into conside-ration.
Thermal resistance and shelf life of the material are critical together
with all typical properties of plastic cards, and last but not least,
the price of the substrate and the cost to convert it into plastic cards.
Some requirements are constraints, therefore a ôcompromiseö is required.
The ingredients
One of the aims is to provide customers with a homogeneous material,
and to totally achieve this target, the suitable PVC grade must be selected;
to improve the mechanical properties, always appreciated by the end
user, we could try to use a PVC with high K value. A choice extreme
would then obligate the manufacturer to increase the percentages of
some additives called "processing aids", with two direct and
unavoidable consequences — worsening of the surface quality of
the sheet and an increase in the cost of the formulation. One alternative
is to modify the inner lubrication of the formulation. With the equilibrium
already delicate and unstable, excessive lubrication of the formulation
can cause a migration to the surface, towards the hot calendering rollers
creating a deposit (plate out) that directly influences the surface
quality of the film. This will also create a reduction in the surface
tension of the material which, in turn, will lead to poor adhesion of
the inks due the presence of wax on the surface of the sheet. Pure PVC
is yellow and the high temperatures of the calendering process dictate
the requirement to stabilize the PVC, the titanium dioxide and pigments.
The implications of stabilizers are limited, especially if the percentage
is limited; more issues are related to the titanium dioxide because
of its cost and density.
Also the pigments are very expensive but the quantities are marginal.
A uniform printing surface isn't enough; physical and mechanical properties
are also required. For the elasticity two options are available: ABS
(Acrilinotrile butadiene styrene) and MBS (Methyl acryline butadiene
styrene) and the choice is made taking into consideration the performance
we want to achieve and the complexity of the formulation.
The good mechanical properties are not unconditional, however, and
in fact there are some unpleasant implications related to the use of
ABS/MBS: lower printability and ink adhesion, colour change, embossing
and character retention, and a lower thermal resistance. To improve
the softness of the material, the solution is the PVCA (Polyvinyl chloride
acetate), better known as copolymer but it has a lower thermal stability
and is more expensive than the homopolymer. This polymer helps all thermal
processes, such as lamination. The flexibility (resilience plus elasticity)
can be achieved using PVC with high K value and some ABS/MBS.
More options are available to improve the thermal resistance of the
material because we can use a special type of PVC, CPVC (Surclorate
PVC). It guarantees the good properties of PVC because of its different
melt viscosity but is more difficult to blend. The second option is
to use AMS-ABS and/or AMS-SAN with the benefit to possibly increase
the percentage without blending issues but these additives do not bring
the advantages of PVC (ink adhesion, mechanical properties, etc.).
To guarantee and improve the aging resistance of core and overlay,
some UV absorbers and light stabili-zers are available which finally
require an adjustment with more pigments to compensate the yellow colour.
If special features are required, we must integrate other additives
into the chemical formulation, and obviously, this can affect the delicate
equilibrium.
The final result is a formulation with a lot of different ingredients,
normally between 10 and 15. Because the equilibrium is so unstable,
the tolerances are very critical. The mix sequence is also important
due to the chemical compatibility, behavior, properties, grain size
and density.
The chemical formulation must take into account some calendering process
requirements. I have already stressed the handling issue but at this
stage I would like to underline the difficulties in handling raw materials
with different grain size and density.
There are limits from the mixing and compounding; the type of PVC is
very important but the additives selection is even more critical, as
is their mixing sequence, and finally the gelation because we aim to
get a homogeneous grade, by melting all ingredients together and at
the same time. Obviously some processing aids and lubricants are required.
Homogeneity means consistent formulation both from chemical composition
and temperature point of view. Because of the high temperature of the
extrusion line normally used to finally melt the ingredients, we must
avoid degradation of the formulation before the calendering process
begins, therefore the formulation is stabilized.
Due to the temperature of the rollers, between 160-200° C, the
mix requires some lubricants and stabili-zers taking into account the
thickness of the film because a 300 micron film (12') absorbs the same
energy a more thick film, for instance a 600 micron film (24'). The
last step of the calendering process is the release of the sheet from
the roller, so to minimize the stress on the film during this step,
some external lubricants are normally used.
The above analysis is purely from the chemical formulation perspective
but, of course, there are a lot of implications due to the calendering
line layout (shape, size and position of the rollers), process parameters
(absolute and relative speeds and temperatures), thickness control,
flatness adjustment and shrinkage control making a formulation even
more delicate to develop. A grade able to meet all these requirements
is the result of complex design and project activity. It's very difficult
to combine everything in one product because the card manufacturing
process and the card requirements are sometimes in contradiction, therefore
the beauty of the formulation is in the ability to create cohabitation
with the opposite ones. But it's possible and the following case should
be a good example:
Project A21-00
One leading card manufacturer decided to adopt for a special application,
a split core of 340 microns for smart cards with cyanocrilate glue,
and they were asking for a very smooth printing surface suitable for
UV litho, Vicat an 84° C at 1 kg (A-50) and a particular white shade
(blue-green), with the material very opaque. The customer defined the
lamination conditions, the peeling strength between the two cores and
between core and overlay, and finally the flex test to pass (see above
table).
As if these factors were not enough, the customer was under extreme
pressure: the final customer was asking for the first sample cards within
45 days! Together we decided to fix some priorities, which in this project
were the mechanical properties, Vicat and lamination conditions; next,
basic guidelines were identified. To achieve the mechanical properties
we decided to use PVC with high K value pure or blended with possibly
some MBS, for the Vicat ABS or CPVC and finally to guarantee an easier
lamination PVC with low K value or a PVC with high K value blended with
PVCA.
Step 1
100 PHR (per hundred resin) of high K value PVC to meet the mechanical
properties blended with 12.5 PHR of PVCA (copolymer) and some ABS to
reach the Vicat, titanium dioxide for the opacity and a special additive
to guarantee the chip embedding with cyanocrilate glue.
+ The grade was good but very easy to delaminate the two cores.
Step 2
In order to improve the softness of the material, we were obliged to
increase the percentage of PVCA to 60 PHR but obviously the copolymer
had a direct effect on the Vicat and to compensate it we had to increase
the ABS.
+ Flex test and lamination were good but the Vicat was too low, at
the bottom end of the limit.
Step 3
The easiest solution to increase the Vicat is to increase the ABS but
the mechanical properties can collapse. Therefore, we took the option
to reduce to 30 PHR the percentage of PVCA (increasing the Vicat but
also reducing the softness of the material compared with the grade developed
in Step 2) and reduce ABS to keep the ratio between the two ingredients
balanced.
+ No lamination.
Step 4
We decided to increase the PVCA to 47 PHR but unfortunately the Vicat
was still at the bottom end of the limit.
+ Vicat too low.
Step 5
Once again an option was an increase in the ABS content but we were
concerned about the mechanical properties so, therefore, we decided
to introduce a new ingredient, CPVC (Surclorate PVC), and thus managed
the difference in viscosity between PVCA and CPVC.
+ The grade was passing all relevant tests.
Fine tuning
In this final step, we adjusted opacity, colour and Vicat respectively
using titanium dioxide and a blue pigment. Some lubricants and antioxidants
were also required.
+ The customer approved the grade and the deadline of 45 days was met.